1. Field of the Invention
The present invention relates to a system for controlling the shift of an automatic transmission and, more particularly, to a shift control system for controlling execution of a so-called "clutch-to-clutch shift".
2. Related Art
A shift effected by releasing a first frictional engagement element and by engaging a second frictional engagement element, is called a "clutch-to-clutch shift". When such a shift is an upshift a transitional "overlap state" occurs in which the two frictional engagement elements temporarily have a torque capacity higher than a predetermined value whereas, when a downshift, a transitional "underlap state" occurs in which the two frictional engagement elements are simultaneously temporarily released. The shift is effected by oil pressure which is controlled by electric signals.
At the time of the aforementioned shift in the prior art, the controlled oil pressure is corrected in subsequent and later shifts by detecting the racing of the engine in terms of the input/output R.P.M. so as to prevent the excessive engine racing, which is caused by the failure of the oil pressure to change, as expected, responsive to the electric signals or by a reduction of the output shaft torque due to tie-up (as disclosed in U.S. Pat. No. 5,018,068).
In the prior art referred to above, however, the correction of the controlled oil pressure is set so that the engine will not excessively race. Since the controlled oil pressure in not corrected when racing of the engine is not detected, the output shaft torque drops and thereby invites shift shock if tie-up occurs.
It is theoretically possible to detect the tie-up so that the correction of the controlled oil pressure may be set on the basis of the detected value for tie-up. However, it is difficult to detect tie-up from the input/output R.P.M. Still worse, detection of tie-up would be costly, and would require a larger system to accommodate a special torque sensor for detecting the torque from the input/output shaft, for example. Further, such an arrangement would require the automatic transmission to be substantially modified.